Automatic Dependent Surveillance-Broadcast (ADS-B) is a cooperative surveillance technique used for air traffic control and related applications. ADS-B-equipped aircraft determine their position using a Global Navigation Satellite System (GNSS) such as the United States Global Positioning System (GPS), or other position-determining equipment. The determined position of the aircraft is then combined with other data and broadcast from the aircraft using an ADS-B transceiver (e.g., a transponder). The transmitted data may include the determined position, the type of aircraft, the speed of the aircraft, the aircraft's flight number, and whether the aircraft is turning, climbing, or descending. Other ADS-B transceivers integrated into the air traffic control (ATC) system or installed aboard other aircraft receive and use the broadcast information, which is periodically updated, to provide users with an accurate depiction of real-time aviation traffic, both in the air and on the ground.
ADS-B is comprised of two different services: ADS-B Out and ADS-B In. ADS-B Out relates to the periodic broadcast of information about the aircraft, such as identification, current position, altitude, and velocity, through an onboard transmitter. ADS-B In relates to the receipt of transmitted data, such as Traffic Information Services-Broadcast (TIS-B) data, Flight Information Services-Broadcast (FIS-B) data and direct communication from nearby aircraft. Ground traffic systems process received ADS-B transmissions and other information to broadcast data related to aviation traffic. For example, ATC systems may broadcast TIS-B data, which includes aggregated ADS-B data received from aircraft transmitters, and additional information, such as FIS-B data, to enable broadcast recipients to have a comprehensive understanding of ground and air traffic. TIS-B broadcasts supplement ADS-B to provide additional situational awareness of all traffic known to the ATC system, including aircraft that are not transmitting ADS-B information. The ATC ground station transmits surveillance target information on the ADS-B data link for unequipped targets or targets transmitting only on another ADS-B link. (e.g., 978 MHz vs. 1030/1090 MHz)